Scanning methods are reviewed in R. K. Molla, Electronic Color Separation, R. K. Printing & Publishing, New York, 1988, the disclosure of which is incorporated herein by reference. The principles of color are explained in G. Wyszecki and W. S. Stiles, Color Science, Wiley & Sons, 1982, the disclosure of which is incorporated herein by reference.
Generally speaking, tone and color reproduction control in high quality graphic arts reproduction is still far from a science. This is particularly evident when a given acceptable result, already realized using given reproduction apparatus, is sought to be realized using other apparatus or using the same apparatus but with a different setting, such as a GCR setting relative to a normal "key black" setting. In such cases, a high degree of expertise, combined with time, effort, expense and patience is required to calibrate the additional apparatus. The results are not always satisfactory.
Unidimensional calibrations in graphic arts, in which a plurality of calibrations are carried out, each being a function of only one color, are known. State of the art techniques include gray balance correction and plotter output calibration techniques. Another example of unidimensional calibration is the automatic TCR (tone and color reproduction) correction process disclosed in published European Application 84307997.1 of Xerox Corporation (Publication number 0144188 A2).
The disadvantage of unidimensional calibrations is that they are only accurate in certain portions of the color space, since a full determination of color is multidimensional, typically having three or four components. For example, the teaching of the above-mentioned published European Application 8437997.1 is relatively inaccurate except in the area of a particular machine's primary color coordinate axes. Gray balance techniques are relatively inaccurate except for a relatively small volume of the color space, comprising gray colors only. Also, the apparatus disclosed in the above-cited published European Application 8437997.1 can be calibrated only by its own output.
Methods of computing a multidimensional function to fit a given set of vectors are known. Interpolative methods may be used if the data is suitably distributed. However the desired conditions regarding the distribution do not always hold in color processing applications, because the data is often not produced directly but rather is the end result of certain procedures (such as scanning, printing, etc.) which are performed on initial preselected data.
An article by Stone et al (Stone, M. C.; Cowan, W. B. and Beatty, J. C., "Color Gamut Mapping and the Printing of Digital Color Images", ACM Transactions on Graphics, 7(4), October 1988, 249-292) discloses use of a colorimeter in mapping a color gamut and printing digital color images, using human aesthetic judgement as a criterion. Human aesthetic judgement is not always the most relevant nor the most efficient criterion for calibrating a color processing device, particularly when it is desired to use another color processing device as a calibration reference. The use of a colorimeter as disclosed by Stone et al is typically cumbersome and time-consuming.
U.S. Pat. No. 4,500,919 to Schreiber discloses a system for producing color reproductions of an image in which an operator may interactively manipulate a display of the scanned image in order to introduce aesthetic, psychophysically referenced corrections therein. Schreiber teaches that it is desirable for such a system to provide automatic compensation for the effects of ink and paper while allowing the operator to input aesthetic alterations.
U.S. Pat. No. 4,719,954 to Fujita et al. describes a method and apparatus for creating a color conversion table between scanned colors of a color chart, typically in the Red-Green-Blue (RGB) color coordinate system, and printable colors, typically in the Cyan-Magenta-Yellow-Black (CMYK) color coordinate system, and for using the color conversion table to reproduce a selected measured color in a color specimen. If the selected measured color does not coincide with a value in the color conversion table, an interpolation step is performed.
The method of Fujita et al also includes a correction step when reproduction is performed under different printing conditions. The correction step compensates for the difference between the two printing conditions.
Image creation systems typically comprise a computer with associated graphic software for generating digital representations of color images and/or modifying digital representations of color images, and a plotter or other color output device For transforming the digital representations into analog representations. The analog representation may be created on any suitable substrate, such as on a dia. If desired, e.g. in pre-press applications, the resulting dia can be scanned.
Examples of commercially available graphic software are Photoshop, by Adobe Systems Inc., Mountainview, Calif., U.S.A., usable in conjunction with the Mac II by Apple Computer Inc., U.S.A.; and PC Paintbrush Plus, by ZSoft, San Francisco, Calif., U.S.A., usable in conjunction with the IBM PC. Examples of commercially available plotters are 4cast, by DuPont, Wilmington, Del., U.S.A., and the LVT Model 1620 digital image recorder by Light Valve Technology, Rochester, N.Y., U.S.A.